The present disclosure relates generally to wireless communication systems, and more particularly to a method for suppressing interference in adaptive beamforming.
Adaptive beamforming is a widely-used technique to achieve maximum capacity and coverage in wireless communication systems. Specifically, adaptive beamforming attempts to configure an away of antennas, in such a way that signals arriving from a desired direction at a specific frequency is detected and accepted while signals arriving from other directions at that specific frequency is rejected. The configuration mechanism typically includes iteratively computing a plurality of weights attributable to their corresponding antennas in the away of antennas to enhance the signal arriving from the desired direction while minimizing or eliminating the signal arriving from other directions. The desired direction is typically found by phasing the feed to each antenna of the away such that signals received or transmitted from all antennas of the away will be in phase in a particular direction.
On average, beamforming gain of 20 log10(N) can be achieved for a downlink communication while a gain of 10 log10(N) can be achieved for an uplink communication, wherein N is the number of antennas in the antenna array. Capacity increase in antenna array beamforming is mainly attributable to power increase, improvement of channel conditions, and suppression of interference. Power is increased by a factor of 2N in downlink communication, and by a factor of N in uplink communication. The direct power increase can be directly translated to a higher modulation order, thereby leading to a higher capacity. An antenna array improves the channel conditions significantly by optimized combination of multiple incoherent signals. Since the order of the modulation and coding rate depend on the channel conditions, better channel conditions means higher order modulation and low coding rate. Furthermore, good and stable channel conditions yield a more efficient link adaptation, thereby achieving a higher capacity. Finally, an antenna away allows the detection and suppression of multi-cell interference, which are critical in networks that require frequency reuse.
For example, modem communication networks are required to be compatible with a regular cellular network but also available to deliver at least 20 Mbps of data communication throughput per cell. The data throughput represents a 140-fold increase compared to the conventional voice rate of 14.4 kbps, assuming that the sustained data rate is 50 kbps, the average time for a voice call is 3 minutes and the average data time is 2 hours (50*60*2/(14.4*3)≈140). If a modulation order QAM16 is supported on average, the spectrum required per sector will be 5 MHz (20/4). If the network operator has 15 MHz spectrum available in total, the highest frequency reuse will be N=3. However, without a means to suppress same-channel interference, the deployment of frequency reuse (N=3) will yield poor system performance, because interference would render a poor signal-to-noise ratio in the majority of the area that even the more inferior QAM4 can support.
To suppress interference, a variation of adaptive beamforming is used. In null-steering beamforming, instead of enhancing the signal arriving from the desired direction, as given above, the direction in which the desired signal is not preferred is examined, and the output signal power pertaining to that direction is minimized.
However, beamforming calculation is a difficult task because interference occurs unpredictably. Traditional beamforming methods either require detecting interference in the entire frequency-time span of the wireless communication band, thereby yielding a good set of detection data but at a high capacity and processing cost, or detecting interference only in selected portions of the entire frequency-time span, thereby reducing the capacity and processing cost but losing overall data quality due to lack of a complete interference picture. There is currently no efficient method for efficiently detecting interference in a null-steering beamforming approach.
What is desired are methods that provide for more efficient detecting and suppressing of interference for optimizing antenna array capacity.